1. Field of the Invention
The present invention relates to a multiple-line image sensor for use in a color image reading device and, in particular, to an image sensor and an image reading device having a function of continuously changing the read sub-scanning density of a digital copying machine or the like, and also having a multiple-line storage means.
2. Description of the Related Art
Conventionally, as a color image reading device, there is widely used a color image reading device which includes, on one chip, a 3-line color sensor having three read lines for three colors R (red), G (green) and B (blue). In the color sensor of this type, image information corresponding to one line in a main scanning direction is read out in such a manner that manuscript information is reduction projected onto the surface of the 3-line color sensor by an image forming lens constituting a reduction optical system. Also, image information in a sub-scanning direction is read out by varying a positional relationship between a formed image and a sensor mechanically by use of suitable mechanical means such as a scanning mirror or the like.
Since the respective three read lines for the three colors, namely, R, G and B, in the 3-line color sensor are arranged at different positions on the sensor chip, they read lines on a document extending in the main scanning direction which lines are located at different positions in the sub-scanning direction. For this reason, a color image is read such that the document surface is read sequentially in time series with mechanical sub-scanning and then a registration correction is performed on readout signals. This registration correction is an operation in which the readout signals that are output in time series are delayed on a line-by-line basis so that read lines of the three colors have information as would be obtained by reading the same main scanning line on the document surface.
However, since the read image is quantized in the sub-scanning direction according to a distance (an amount of feed) of the read image to be moved in the sub-scanning direction during one read period (exposure time), the registration correction operation by means of the above delay scanning can only provide accuracy corresponding to an integer-multiple of the feed amount. A residual registration error may cause a case that a black fine line cannot be recognized properly in a digital color copying machine. This error can be eliminated if a read line gap is set at an integer-multiple of the feed amount in the sub-scanning direction. However, in the digital color copying machine, because reduction and enlargement of an arbitrary magnification is performed by changing the scanning density in the sub-scanning direction, the feed amount in the sub-scanning direction varies with the set magnification, which leaves a problem when it is intended to accommodate all magnifications.
FIG. 29 shows a general configuration of an image reading device using a 3-line color sensor, and FIG. 30 illustrates how color misregistration in the sub-scanning direction occurs in this 3-line color sensor.
In the image reading device using the 3-line color sensor, as shown in FIG. 29, to read an image of a document placed on a document placement glass 201, a reflection image of the document is reduction-projected progressively onto a 3-line color CCD sensor 205 through a lens 204 with scanning by a full-rate mirror 202 and half-rate mirrors 203. In this image reading device, for example, the pixel pitch of the sensor is 14 .mu.m, the document read resolution of the sensor is 16 dots/mm, and the projection magnification of the lens is 0.224 (=0.014.times.16). In the conventional 3-line color CCD sensor, the read pixel lines R, G and B are arranged in parallel to one another and, therefore, as shown in FIG. 30, different positions on the document surface are read. If the gap between the sensor pixel lines is 168 .mu.m, positions on the document surface which are spaced apart by 0.75 mm (=0.168/0.224) are read.
In the scanning by the full-rate mirror 202 and half-rate mirrors 203, since the lens 204/sensor 205 system is moved at a constant speed with respect to the document to be read and thus the document information is read and scanned relatively, different pieces of document information on the surface of the document placement glass are read out as the data of different read times by the 3-line color CCD sensor 205. For this reason, this registration correction can be made by delaying the outputs of the sensor on a line-by-line basis by use of a line FIFO of a color registration correction memory 206.
Next, there is shown a general formula to determine the number of delay lines necessary for a line gap correction that is needed when the read magnification is changed.
Del=Gap.times.Res.times.Mag PA1 Del: number of delay lines; PA1 Gap: three-color read line gap as converted a value measured on a document surface (in a conventional case, the gap between R and G as well as between G and B is 0.75 mm, and the gap between R and B is 1.50 mm); PA1 Res: read resolution (in the conventional case, 16 dots/mm); and PA1 Mag: read magnification (1.0 for 100%).
where
TABLE 1 ______________________________________ Read magnification 100% 97.9% 95.8% 93.8% 91.7% ______________________________________ Number of delay 24 23.5 23 22.5 22 lines of R with respect to B Number of delay 12 11.75 11.5 11.25 11 lines of G with respect to B ______________________________________
Table 1 shows how the number of delay lines varies for magnification values around 100%. The number of delay lines varies according to the read magnification as shown in Table 1, and thus it is not always an integer. To correct this error, it may be conceivable to equalize the read centers of gravity of the three colors by using an average of data of lines adjoining a read line. However, since the operation to average data of the adjoining images is a blurring operation, the read resolution is deteriorated. Further, since the read resolution is deteriorated only in the color to be subjected to registration correction, a resolution balance between the three colors is lowered. This may disable proper recognition of a black fine line.
FIG. 31 shows an example of black fine line read data of the respective color sensors in a case where the delay amount does not correspond to an integral number of lines, and FIG. 32 shows an example of the read correction by delaying as well as a change of the center of gravity by moving average. As shown in these figures, as a result of execution of the center of gravity correction operation, the read image of G situated in the center of the 3 lines becomes unsharp and, as shown in FIG. 32, although they are the read data of the black fine lines, the values of the center portions thereof and the values of the edge portions thereof are not coincident in the three color read data. This results in coloration and muddy edges. As a countermeasure against this problem, Japanese Patent Examined Publication Nos. Hei. 6-81225 and Hei. 6-20221 have proposed a technique in which the registration correction of decimal portions is effected by independent operations in which the exposure periods of the three-color read lines are deviated from one another. The present inventors have also made a similar proposal in Japanese Patent Application No. Hei. 7-10400.
Now, FIGS. 33A and 33B show examples of shift pulse input timings to the 3-line color CCD sensor and exposure phase timings variable according to the shift pulse input timings. The exposure period timings of the three colors can be changed by the supply timings of shift pulses that control the exposure period of the 3-line color CCD sensor. In an exposure timing control example of FIG. 33A, the number of correction lines between R and G and between G and B is n+1/2. In an exposure timing control example of FIG. 33B, the number of correction lines between R and G and between G and B is n+1/4 while the number of correction lines between R and B is 2n+1/2.
Now, FIG. 34 is an explanatory view of a conventional 3-line color CCD sensor, FIG. 35 is an explanatory view of a 3-line color CCD sensor of an intra-pixel transfer system, FIG. 36 is a block diagram of a reading device using the conventional 3-line color CCD sensor, FIG. 37 is a block diagram of a reading device using the 3-line color CCD sensor of an intra-pixel transfer system, and FIG. 38 is a graphical representation which shows a relationship between the sizes of line sensor gaps and the amplitudes of allowable speed variations. In the conventional 3-line color CCD sensor, for example, as shown in FIG. 34, horizontal transfer electrodes are arranged on both sides of their respective light-sensitive pixel lines. On the other hand, in the intra-pixel transfer system, for example, as shown in FIG. 35, the light-sensitive pixel lines of the three lines are arranged such that they adjoin one another, and their horizontal transfer electrodes are disposed on the outside thereof. As shown in FIGS. 36 and 37, the signal charges of the respective three colors are taken out from the horizontal transfer electrodes in such a manner that they are classified into Odd and Even signal charges.
By the way, the 3-line color registration correction in the 3-line color sensor is made in such a manner that the shifted read time is substituted for the shifted read position and then the time is delayed in a unit of lines by the memory. However, to achieve this correction, it is necessary that the feeding in the sub-scanning direction is a uniform linear motion of a high accuracy. Here, if the gap distance between the read pixel lines is great, then the delay time is long and is thus easy to be influenced by variations in the feeding speed of a long cycle, which requires a further accurate sub-scanning direction feeding motion. For this reason, it is desirable that the gap between the read pixel lines is narrower, more preferably, the sensor line gap is in the range of 1 to 2 lines. As can be seen clearly from FIG. 38, if the sensor line gap is less than 2 lines, the allowable range extends rapidly.
However, in the structure of the conventional 3-line sensor shown in FIG. 34, since the horizontal transfer electrodes are arranged as read-out electrodes on both sides of the light-sensitive pixel lines of the respective colors, there arises a problem when the gap between the read pixel lines is reduced down to the range of 1-2 lines. Also, there can be expected another means in which wires are provided between the pixels for taking out the output of the central pixel line of the three pixel lines. In this means, however, the areas of the pixels are reduced and thus the sensitivities of the pixels are decreased down almost to a half, which is insufficient for the specifications which require high speeds, high resolution and low noise.
Techniques for narrowing the sensor gap are proposed in Japanese Examined Patent Publication No. Hei. 6-65230 and Japanese Unexamined Patent Publication No. Sho. 63-191467. These publications employ a system in which the signal charges of the inside photosensitive pixel lines of the respective photosensitive pixel lines of the 3-line sensor are read out through the outside photosensitive pixel lines. This system is referred to as an intra-pixel-line transfer system. That is, in these publications, by using this intra-pixel-line transfer system, the sensor line gap of the center line signals can be reduced down to the range of 1-2 lines through between the pixels without reducing the pixel areas of the sensor to lower the sensitivities of the pixels. An example employing this structure is shown in FIG. 35.
However, in the intra-pixel-line transfer system, since the read signal charges of the center pixel lines of the photosensitive pixel lines of the three lines are read out through the outside pixel lines, it is difficult to employ the above-mentioned exposure timing shifting technique. That is, the signal charges of G are sent from the photosensitive pixel lines of G situated in the center of the three lines through the R photosensitive pixel lines to the horizontal transfer electrodes of G. For this reason, if the exposure timings of R and G are shifted, it interferes with the read-out passage of G during R exposure.
Thus, because the exposure timing control is impossible, the gap between the light-sensitive pixel lines is set to be the integral multiple of the scanning density (which is normally determined by the gap between the pixels in the main scanning direction). This is because, when the scanning magnification is 100% which is the most standard magnification, the gap correction is made only on the integral lines, which eliminates the fear that the resolutions due to the registration correction error and registration correction can be lowered. Due to this restriction, though a photosensitive pixel line gap of less than 2 lines can be technically realized, the actual gap of the pixels themselves in the sub-scanning direction cannot help but 2 lines. There can also be employed a technique in which, by shortening the length of the photosensitive window of the pixel line in the sub-scanning direction, the pixel line gap in the sub-scanning direction can be set as 1 line. However, in this case, the sensor sensitivity is lowered. Further, in the conventional technique, there arises a problem that variations in the magnification cannot be read freely.